1. Field of the Invention
The invention relates to a highly machinable, hot-isostatically-compacted, prehardened, martensitic steel article used for molds for plastic injection molding, metal die casting die components, and other hot work tooling components, and to a method for producing the same.
2. Discussion of the Related Art
Typically, molds for plastic injection molding, components for die casting dies, hot forging dies, extrusion dies, and other hot work tooling, henceforth referred to as molds and dies, are manufactured by rough machining the component close to finish dimensions from a hot work tool steel mold or die block, hardening the rough-machined component by a quenching and tempering type of heat treatment, and finally machining the hardened component to finish dimensions. During the heat treatment process, dimensional changes occur in the rough-machined component as a result of metallurgical phase changes in the mold or die block, and as a result of nonuniform thermal stresses which are inherent in the heat treatment process. These dimensional changes cause the need for a second, finish machining operation to be performed after heat treatment to machine the component to its final shape and dimensions. The required second machining operation results in increased costs and manufacturing time in the construction of a mold or die.
The use of a prehardened mold or die block eliminates the need for a second, finish machining operation. Prehardened mold and die blocks made from conventional, resulfurized, AISI H13 hot work tool steel are currently available. The sulfur additions in the steel make it machinable at the high hardness needed for mold and die applications (35 to 50 HRC), but components manufactured from the currently available, prehardened mold and die blocks exhibit low toughness and reduced service life because the sulfur addition reduces the notch toughness of the steel. In addition, the sulfide particles which form in the steel degrade the ability of a mold block to be polished to the high-quality surface finish that is required in many plastic injection molding applications.
Because of the low notch toughness and reduced polishability exhibited by the currently available, prehardened mold and die blocks, their use is not wide-spread in the plastic injection molding and hot work tooling industries. Their low notch toughness is a significant disadvantage that reduces their usage because of the high costs and safety hazards incurred if a mold or die fractures during service. Thus, the potential industry-wide cost savings which could result from the use of highly machinable, prehardened die and mold blocks is not realized. A need therefore exists for highly machinable, prehardened, martensitic tool steel mold and die blocks that can be used without sacrificing tooling performance and longevity.
Additional cost savings can be realized by manufacturing mold and die blocks from hot-isostatically-compacted compacts of prealloyed powder of suitable chemical compositions. Such compacts are manufactured by placing the prealloyed powder into appropriate containers, sealing the containers with an air-tight seal, and then subjecting the containers to various combinations of high temperature and pressure. Because compacts so produced achieve 100 percent of theoretical density, there is no need for additional thermomechanical treatments such as hot forging, hot rolling, or hot extrusion to manufacture the mold and die blocks. Elimination of these thermomechanical treatments results in substantial savings in costs, manpower, and energy usage in the manufacture of prehardened mold and die blocks.
In addition, the production of prehardened mold and die blocks by heat treatment of hot-isostatically-compacted compacts results in unique metallurgical and mechanical properties in the mold and die blocks so produced. Specifically, because there is no plastic deformation from subsequent thermomechanical treatments, second-phase particles which form in the steel remain essentially spherical and there is no elongated grain structure in the steel parallel to a hot working direction. These features result in mechanical properties which are essentially isotropic, which in turn eliminate the need for mold and die manufacturers to judiciously choose the orientation of the steel to anticipate the orientation of stresses in the finished mold or die component.
Further, by producing prehardened mold and die blocks by hot isostatic compaction, without additional thermomechanical treatments, near-net-shape mold and die blocks can be manufactured. Custom-shaped containers can be fabricated to produce a mold or die block of a specific shape that minimizes the need for extensive machining after hot isostatic compaction and heat treatment. This further reduces the costs, manpower, and energy requirements in the manufacture of mold and die components.